Sand which has been contaminated with hydrocarbons such as oil or other contaminants must generally be processed to remove the contaminants before the sand may be used in most industrial or agricultural applications. The processing of contaminated sand can be an expensive and time consuming operation.
Canadian Patent No. 2,319,566 to Bacon et al. discloses a process for separating oily films from sand particles carried as a slurry in a solvent-free water stream. The process disclosed by Bacon et al. utilizes scrubbers for detaching oil from sand and specially designed reactors for subsequent removal of oil from the slurry.
In practice, scrubbers have a highly detrimental emulsifying effect. Scrubbers break down oil particles that are bonded to sand granules into minute particles that form emulsions. Means for removing minute oil particles from water are generally ineffective. Emulsified oil contained by slurry migrates downstream and contaminates the sand product. Furthermore, a portion of the emulsified oil accumulates in water tanks and gradually contaminates the elutriation water utilized for sand rinsing.
Due to excessive agitation caused by scrubbers, solids-water separation in the reactors of Bacon et al. is critically hindered. As a result, sand may be carried over from the reactors and block surge tanks with deposits. Fines originating from the reactors may remain in suspension and tend to prematurely erode the jet pumps of Bacon et al. deposits. Fines originating from the reactors may remain in suspension and tend to prematurely erode the jet pumps of Bacon et al.
Jet pumps employed for transferring sand between adjacent reactors in the process of Bacon et al. also emulsify oil and contribute to water and sand contamination. For optimal operation jet pumps pump a slurry that typically contains eight parts of water for two parts of sand. The relatively large quantity of water in the slurry requires relatively large reactors, dewatering devices and auxiliary equipment. As a result, capital and operating costs are increased significantly.
Furthermore, dewatering devices used for extracting the sand product from the slurry in the process of Bacon et al. experience various problems in handling large volumes of water. Such problems may include, premature shaker screen erosion, excessive sand carry over from screw hoppers, frequent failure of the decanter centrifuge coupling and erratic hydrocyclone operation.
More importantly, the inventor has determined that the operation of the jet pumps in the process of Bacon et al. cannot be effectively controlled to maintain a layer of sand at the lower region of the reactor. The sand layer normally acts as a barrier for contaminants. The loss of the sand layer occurs rather regularly and may result in unrestricted passage of contaminants through the reactors and contamination of the sand product.
There exists a need for methods and systems for washing sand which may be used to produce substantially contaminant-free sand without consuming large volumes of water.